1. Field of the Invention:
This invention relates to a dielectric ceramic composition used in making electronic devices. More particularly, it is a composition which comprises compounds having the perovskite structure and is particularly useful in making devices used in the SHF range.
2. Description of the Prior Art:
There are known dielectric ceramic compositions which are used in making electronic devices because of their small loss of power and their good temperature characteristics. Those devices include a temperature compensating capacitor, a dielectric resonator for the direct reception of satellite broadcasting, or for a down converter, in which those compositions are used because of their small loss of power in the SHF range, and a substrate for a microwave strip line.
While many compounds having the perovskite structure are used for dielectric ceramic compositions which are employed in making devices used in the SHF range, the compounds which are used particularly widely are of the formula: EQU Ba(B.sub.1/3.A.sub.2/3)O.sub.3
where A is Ta, and B is a bivalent metal ion (one or more of Zn, Ni, Co and Mn). More specifically, the compound of the formula Ba(Zn.sub.1/3.Ta.sub.2/3)O.sub.3 is used above all.
The dielectric ceramic compositions which are intended for use in the SHF range are strictly required to have a high dielectric constant .sup..epsilon. r, a high unloaded Q value and a temperature coefficient of resonant frequency .sup..tau. f, of 0. These requirements call for the precise control of the composition of any such material which in turn necessitates a long time of sintering amounting to, say, 100 hours at a temperature of 1500.degree. C.
It is particularly important to control the proportion of Zn accurately, as it is very likely to evaporate. Zn diffuses to the surface of a ceramic and volatilizes during a sintering operation, resulting in the formation of a compound not containing any Zn, such as Ba.sub.5 Ta.sub.4 O.sub.15. This compound forms a "skin" and thereby makes it difficult to produce any ceramic having uniform quality and stable characteristics.
A long time of sintering at a high temperature is required for producing a material having a satisfactorily high unloaded Q value. The necessity for a long sintering time is an obstacle to the mass production of any material under consideration.
For these and other problems, reference is made to J. Am. Ceram. Soc., 68 [10], 546-51 (1985).
Moreover, some applications call for a material having a specifically controlled temperature coefficient, .tau.f, at the resonant frequency. It is, however, known that the material of the formula Ba(Zn.sub.1/3.Ta.sub.2/3)O.sub.3 has a temperature coefficient in the vicinity of zero at the resonant frequency.